The present invention related generally to high frequency probing systems and more particularly to a high frequency probing system having electrical over stress (EOS) and electrostatic discharge (ESD) protection.
Ultra high speed sampling heads used in time domain reflectometry typically dictate extremely low parasitic capacitances. This introduces unique problems. Sampling devices are much more sensitive to static charges residing on a device under test before a test probe touches it. The small geometry of the sampling diodes in the sampling heads often dictate low breakdown voltages. The low parasitic capacitance at the sampling head input means that for a given device under test (DUT) static charge, there will be a higher transient voltage at the sampler input because of the reduced charge sharing effect. It is therefore important to neutralize any static charge on the device under test before the sampling head input is coupled to the device under test.
Conventionally, users are advised to take all anti-static precautions including purchasing and installing antistatic equipment and employing anti-static procedures. Such equipment and procedures include using ionized airflow devices to reduce the DUT static charge on isolated conductors, ground straps on the test bench and the operator, and an anti-static mat around the test bench. Another piece of anti-static equipment that may be used with sampling heads is the SIU600 Static Isolation Unit, manufactured and sold by Tektronix, Inc., Beaverton, Oreg. or the Model 1201 Static Isolation Unit, manufactured and sold by Picosecond ATE, Inc. Beaverton, Oreg. Referring to FIG. 1, there is representatively shown the static isolation unit 10 that includes an interface box 12, foot pedal 14, and a power adapter 16. The power adapter 16 is connected to a standard electrical outlet to provide DC power to circuitry within the interface box 12. An RF probe 18 for probing a device under test 20, such as circuit runs 22 on a circuit board 24, is connected to the interface box 12. A coaxial cable 26 couples the interface box 12 to a TDR sampling head 28 mounted in a sampling oscilloscope 30. The foot pedal 14 is connected to the interface box 12 for coupling the output of the device under test 20 through the interface box 12 to the sampling head 28. When the foot pedal 14 is in the normal position (not pressed), the input of a buffer circuit is coupled to a TTL logic high that cuts off current flow in a drive circuit to an RF relay in the interface box 12. The normally open RF relay coupled the probing tip of the RF probe 18 to electrical ground through a 50 xcexa9 termination resistor 32. Positioning the probing tip of the RF probe 18 on the DUT 20 discharges any static charge stored in the DUT 20. Pressing the foot pedal 14 closes a low resistance switch and allies a TTL active low signal to the buffer circuit that activates drive circuitry in the interface box 12 that energizes the relay and connects the probing tip of the RF probe 18 to the sampling head input, allowing a measurement to be made. The circuitry in the interface box 12 operates under TTL active low logic allowing the foot pedal 14 to be replaced with a TTL external source. The use of TTL active low logic requiring the use of a low resistance switch in the foot pedal 14 for proper operation of the interface box 12 circuitry.
Proper use of the static isolation unit 10 prevents ESD and electrical over stress (EOS) static charge from damaging or destroying the sampling head. The main difference between ESD and EOS is that EOS can occur at a much lower voltage level that ESD. ESD static voltages are typically several hundred to several thousand volts, whereas EOS static voltages may be as low as 15 to 30 volts. The sampling diodes in the sampling head has a breakdown voltage of approximately 9 volts. EOS static discharge causes microscopic damage to the semiconductor layer of the sampling diodes in the sampling head providing a leakage current path around the semiconductor Schottky junction. Over time, the incremental damage of each occurrence of the EOS static discharge continues to degrade the performance of the semiconductor device until the leakage current causes excessive measurement error.
In TDR measurements of a device under test, an operator places the RF probe on the test point with operator""s foot off of the foot pedal 14. The probing tip of the RF probe is coupled to electrical ground through the interface box. Once the operator has properly placed the probe on the test point, the operator depresses the foot pedal with his or her foot and circuitry in the interface box couples the probing tip of the RF probe to the sampling head circuitry. After the measurement is made, the operator removes his or her foot from the foot pedal before removing the RF probe from the test point to disconnect the probe from the sampling head and reconnect the probe to electrical ground. However, in a production environment where repetitive probing is done by the operator, an operator may accidentally keep the foot pedal 14 depressed while repositioning the probe or moving the probe from one test point to another. This allows ESD and EOS voltages on the device under test to be coupled to the sampling head causing damage to the sampling diodes.
One solution is to move the low resistance switch in the foot pedal into the RF probe 18. This would result in a bulkier probe design requiring the placement low resistance switch in the probe along with a hand operated mechanical actuator to allow an operator to activate the switch for measurements. Such a design does not eliminate the possibility of an operator inadvertently keeping the low resistance switch closed while moving from one test point to another.
What is needed is a fail-safe electrostatic discharge and electrical over stress static discharge solution that prevents electrostatic discharges and electrical over stress static discharges from a sampling head input. Activation of the EOS and ESD protection should be incorporated into the measurement probe thus eliminating the need for a foot pedal. The measurement probing system should automatically provide EOS and ESD protection for the sampling head and signal connectivity to sampling head through the ordinary use of the probe for making measurement. The measurement probing system should also allow the use of TTL logic signals for automated measurement applications.
Accordingly, the present invention is a measurement probing system having electrical over stress and electrostatic discharge protection for input circuitry of the measurement test instrument. The measurement probing system has a measurement probe that provides electrical over stress (EOS) and electro-static discharge (ESD) protection control and a control module providing EOS/ESD protection for the input circuitry of the measurement test instrument. The measurement probe has a housing in which are disposed a spring loaded coaxial probe assembly and a pressure sensor. The housing has an internal cavity extending the length of the housing with the cavity exposed at opposing ends of the housing. The spring loaded coaxial probe assembly is formed from a semi-rigid coaxial cable having a center signal conductor, an intermediate dielectric material surrounding the central signal conductor and an outer shielding conductor surrounding the dielectric material. A portion of the outer shielding conductor and dielectric material is removed from one end of the semi-rigid coaxial cable forming a probing tip. The other end of the semi-rigid coaxial cable receives a threaded connector for coupling the semi-rigid coaxial cable via a coaxial cable to the control module. The coaxial probe assembly is disposed in the housing cavity with the probing tip extending from one end of the housing and the threaded connector extending from the other end of the housing.
The pressure sensor has first and second electrically conductive contacts with the first electrically conductive contact secured to and electrically coupled to the outer shielding conductor of the semi-rigid coaxial cable. The second electrically conductive contact is positioned within and secured to the housing. The housing is movable from a first position to a second position relative to the spring loaded coaxial probe assembly from pressure applied to the probing tip of the measurement probe in contact with a device under test.
The control module is preferably mounted in and electrically coupled to the measurement test instrument. The control module has a plurality of coaxial connectors with each coaxial connector having a signal conductor. The signal conductor of a first of the coaxial connector is coupled via the coaxial cable to the central signal conductor of the semi-rigid coaxial cable. The signal conductor of a second coaxial connector is coupled via a coaxial cable to the input circuitry of the measurement test instrument and the signal conductor of a third coaxial connector is coupled via a termination resistor to electrical ground. An input signal connector has at least a first electrical contact that is electrically coupled by an electrical conductor to the second electrically conductive contact of the pressure sensor. The electrical contact of the input connector and the signal conductors of the coaxial connectors are coupled to control circuitry for selectively coupling the probing tip of the semi-rigid coaxial cable to electrical ground via the termination resistor when the housing is in the first position and the control circuitry responsive to at least an activation signal generated by the first and second electrically conductive contacts of the pressure sensor contacting each other by movement of the housing to the second position to couple the probing tip to the input circuitry of the measurement test instrument.
The electrical conductor coupling the pressure sensor to the control circuitry is preferably an insulated electrical wire divided into first and second wire segments. The first wire segment electrically couples the second electrically conductive contact of the pressure sensor to an electrical contact of an electrical connector receptacle mounted on the measurement probe. The second wire segment electrically couples an electrical contact of a first electrical plug to an electrical contact of a second electrical plug with the first electrical plug mating with the electrical connector receptacle mounted on the measurement probe and the second electrical plug mating with input signal connector of the control module.
The control circuitry has a high input impedance transconductance device coupled to receive at least the first activation signal. The high input impedance transconductance device is preferably a p-channel MOSFET that generates an output current for driving a relay that couples the signal conductor of the measurement probe to the input circuitry of the measurement test instrument in the presence of the activation signal and couples the signal conductor of the measurement probe via the termination resistor to electrical ground in the absence of the activation signal. Alternately, the high input impedance transconductance device is a CMOS logic gate that drives a power circuit for driving the relay. The input connector of the control module preferably has first and second contacts with the first contact receiving the activation signal from the measurement probe and the second contact receiving an activation signal from an automated test system.